1. Field of the Invention
The present invention relates to an image heating device applicable to a copying machine, a printer, and other image formation apparatuses. More particularly, the invention relates to an image heating device that causes a film to generate heat by means of electro-magnetic induction, and to a film to be used for such image heating.
2. Related Background Art
As a device using the conventional electro-magnetic induction heating method, there has been proposed in Japanese Patent Publication No. 5-9027 a device structured so that an alternating magnetic field causes eddy current to be generated in a core portion of a fixation roller serving as a heating member to give heat to the core portion with Joule's heat.
In conjunction with FIG. 9, this proposed device will be described. In FIG. 9, a reference numeral 50 designates a cylindrical fixation roller formed by a ferromagnetic material, which is heated by means of induction heating. The magnetic field indicated by arrows shown in broken line in FIG. 9 is generated by applying high frequency alternating current to an excitation coil 52 wound around an excitation iron core 51 to generate an eddy current on the fixation roller 50, to thereby perform heating.
In other words, the eddy current is generated on the fixation roller by means of magnetic flux to cause the fixation roller 50 itself to be heated by Joule's heat. A reference numeral 53 designates an auxiliary iron core for forming closed magnetic path and arranged to face the excitation iron core 51, interposing the fixation roller 50 between them. Also, a reference numeral 54 is a pressure roller having elasticity, which is pressurized to the fixation roller 50 by pressurizing means (not shown), a fixation nipping to form a fixation nipping portion N as a heating portion for thermally fixing a non-fixation toner image T on a recording member P.
The device using the electro-magnetic induction heating method, such a device shown in FIG. 9, can directly heat the fixation roller 50 as a heating member, and arrange its heating position near a non-fixation toner image. Therefore, it is possible to increase the efficiency of energy consumption more than a heating roller using a halogen heater.
However, this device has to heat the fixation roller having a great capacity heat as in a device using a heating roller. Also, the heat efficiency of the device is not enough because of heat dissipation into the interior of the roller. As a result, even when the device has an optimal heat efficiency, it is impossible to carry out quick starting. Also, when Joule's heat is generated by causing a cylindrical member to generate the eddy current, the temperature of the excitation coil and the excitation iron core in magnetic field generating means are increased thus reducing the quantity of magnetic flux. As a result, the heat generation becomes unstable. Also, if temperature rise is great, the excitation coil will deteriorate.
Under such circumstances, the applicant hereof has proposed an electromagnetic induction heating device that enables a film to generate heat, in U.S. patent application Ser. No. 08/323,789. Even with this device, however, it was not enough to thermally fix a thick non-fixation toner layer of maximum four color toner layers laminated in a color image formation apparatus, for example.